As a vacuum drying apparatus for obtaining a powder by continuously removing volatile components from a feed liquid or slurry containing solid components (e.g. polymer), there is known, for example, an apparatus as shown in FIG. 3 (see, for example, Japanese Utility Model Publication No.28862/1977, Japanese Utility Model Publication No.16161/1981 and Japanese Patent Publication No. 38272/1977).
In this apparatus, a feed liquid 2 in a feed liquid chamber 1 is continuously sent to a long tube heater 4 by a pump 3 and heated therein; then, the feed liquid 2 is introduced into a vacuum chamber 5 from the side and subjected to vacuum evaporation, whereby a powder is produced.
In the above apparatus, the amount of solvent remaining in the powder obtained can be made very small (e.g. 1% by weight or below) by appropriately selecting such conditions as temperature, flow rate, vacuum and the like. However, the particles constituting the powder obtained become agglomerate when a feed liquid containing a polymer, etc. is used and takes various forms (e.g. an agglomerate having a pulverized stone shape) when a slurry containing a polymer, etc. is used. When in the above conventional apparatus the feed liquid is sprayed from a nozzle, the resulting powder always comprises scaly particles of, for example, 5 .mu.m in thickness, 20 .mu.m in width and 200 .mu.m in length and it is impossible to obtain a powder comprising spherical particles having an average particle diameter of 500 .mu.m or below and containing a residual solvent in an amount of 1% by weight or below (these conditions are required in order for a powder to be preferably used in a powder coating, etc.).
Hence, the present invention was completed in order to solve the above-mentioned problems of the prior art.
An object of the present invention is to provide a vacuum drying method capable of obtaining a powder containing a residual solvent in an amount of, for example, 1% by weight or below and comprising spherical particles having an average particle diameter of 500 .mu.m or below, as well as an apparatus therefor.
Further objects and advantages of the present invention will become apparent from the following description.